Given: ABCD is a parallelogram and AC bisects BD.
Prove: AB is congruent to BC.

Let's see
In ∆ABE and ∆CBE
So by
SAS congruence the triangles are congruent
AB=BC
Fact:-
It's already given AC is perpendicular to BD
According to general property of rhombus this parallelogram is also a rhombus.
So sides are equal hence AB =BC